The present invention relates to a process for producing an acoustic carbon diaphragm made of carbonaceous materials. More particularly, the invention relates to a process for producing an acoustic carbon diaphragm of carbonaceous materials having a light weight, a high elasticity, a fast sound transmission velocity and an excellent rigidity as compared with a conventional diaphragm material used as a speaker and a microphone, less deformation by an external force, small distortion of sound, wide reproducing sound range, distinct sound quality, adapted for a digital audio age.
It is generally desired to satisfy as a diaphragm for a speaker and a voice coil bobbin the following conditions.
(1) small density, PA1 (2) large Young's modulus, PA1 (3) large propagating velocity of longitudinal waves, PA1 (4) adequately large internal loss of vibration, and PA1 (5) stable against variation in the atmospheric conditions, no deformation nor change of properties. PA1 (1) a method for carbonizing to solely graphite a resin sheet or film, PA1 (2) a method for shaping and carbonizing to graphite a composite material of resin and various carbonaceous powder, and PA1 (3) a method for carbonizing to graphite carbon fiber-reinforced plastic.
More specifically, the material for the diaphragm is required to have a wide reproducing sound range to be reproduced in high-fidelity over a broad frequency band. To efficiently and distinctly produce sound quality, the material should have high rigidity, no distortion such as creep against external stress as well as a large sound propagating velocity. In order to further increase the sound velocity from the equation of EQU V=(E/.rho.)1/2
where V: sound velocity, E: Young's modulus, .rho.: density. the material which has small density and high Young's modulus is obtained.
The materials use paper (pulp), plastic, and further contain glass fiber, carbon fiber compositely mixed with the basic material of them, or processed to metal of aluminum, titanium, magnesium, beryllium, or boron, metal alloy, metal nitride, metal carbide, or metal boride. However, the paper, plastic and their composite materials have small Young's modulus and small density. Thus, the sound velocities of these materials are slow. Vibration division occurs in a specific mode and the frequency characteristics in the high frequency band of the materials are particularly low, resulting in a difficulty in producing distinct sound quality. In addition, these materials are feasibly affected by the external environments such as temperature, and moisture, causing deterioration in the quality and ageing fatigue, thereby disadvantageously decreasing the characteristics. On the other hand, when the materials employ metal plates of aluminum, magnesium, titanium, the sound velocities of the materials are faster than paper or plastic, but since the materials have small E/.rho. value and small internal loss of vibration, the materials have sharp resonance phenomenon in high frequency band or ageing fatigue such as creep occurs in the materials, thereby disadvantageously deteriorating the characteristics. The beryllium or boron provide excellent physical properties. Squawkers or tweeters which use the materials as the diaphragms extend in reproducing limit to audible frequency bands or higher, thereby correctly producing natural sound quality without transient phenomenon by the signals in the audible band. However, these materials are less as resources, and very expensive, and have difficulties in the industrial machining. These processes are difficult to produce speakers of large size.
In addition to these materials, there is a trial to obtain the diaphragms made of carbonaceous material due to large E/.rho. value of carbon materials. That is, there are:
Since the method (1) has small carbon yield of used plastic material, a precise product is not only hardly obtained, but a product having high Young's modulus like graphite or carbon fiber cannot be obtained due to carbon made of plastic.
The method (2) can obtain a production having high Young's modulus as compared with the method (1) by using graphite or carbon fiber, but since it uses various resin so as to improve the moldability, the ratio of the resin carbon to the calcined material is large to cause the Young's modulus of the carbon fiber or graphite to decrease.
Since only the plastic portion is baked and contracted in the method (3) when the carbon fiber-reinforced plastic is calcined, numerous fine cracks occur among carbon fibers so that a product in which the carbon fiber and the resin carbon are integrated without defect cannot be obtained. Therefore, it has such a drawback that the function of the carbon fiber is lost.